Magnetoresistive element, method for manufacturing the same, and method for forming a compound magnetic thin film

ABSTRACT

The invention provides a magnetoresistive element in which the pinned magnetic layer includes at least one non-magnetic film and magnetic films sandwiching that non-magneticfilm, and the magnetic films are coupled with one another by magnetostatic coupling via the non-magnetic film. This element has an improved thermal resistance. Furthermore, the invention provides a magnetoresistive element in which the pinned magnetic layer is as described above. The magnetic films can be coupled with one another by magnetostatic coupling or antiferromagnetic coupling generating negative magnetic coupling. In this element, the magnetic field shift is reduced. Furthermore, the invention provides a magnetoresistive element in which at least one of the magnetic layers sandwiching the intermediate layer includes an oxide ferrite having a plane orientation with a (100), (110) or (111) plane. A magnetic field is introduced in a direction of the axis of easy magnetization in the plane. This oxide can be formed by sputtering with an oxide target while applying a bias voltage to a substrate including a plane on which the oxide ferrite is to be formed so as to adjust the amount of oxygen supplied to the oxide ferrite from the target.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to magneto-resistive elements usedin reading heads of magnetic recording devices such as for optomagneticdisks, hard disks, digital data streamers (DDS), or digital VCRs, inmagnetic sensors for detecting rotation speed, and in magnetic randomaccess memory (RAM).

[0003] 2. Description of the Related Art

[0004] CPP (current perpendicular to the plane)-GMR elements aremagnetoresistive elements using spin-dependent scattering betweenferromagnetic layers sandwiching a conductive intermediate layer,whereas TMR elements are magnetoresistive elements using spin tunnelingbetween ferromagnetic layers sandwiching an extremely thin insulatingintermediate layer. In both elements, the current flows perpendicular tothe film surfaces of the multilayer structure. In these elements, toincrease the reproducibility of changes of the magnetizationdisplacement angle, one of the ferromagnetic layers may be devised as apinned magnetic layer on which an antiferromagnetic layer such as FeMnor IrMn is layered. Also, if a layered ferrimagnetic structure includingantiferromagnetic coupling, for example Co/Ru/Co, is layered on theantiferromagnetic layer, then the pinning magnetic field of the pinnedmagnetic layer can be increased even further.

[0005] Half-metals in which the spin polarization is expected to be 100%by band calculation have garnered attention as ferromagnetic materials.In particular for TMR elements, the magnetic resistance change ratio ishigher, the higher the spin polarization of the ferromagnetic materialis.

[0006] A high thermal resistance is required when the magneto-resistiveelement is applied to magnetic heads, MRAM memory elements or the like.For example, if the TMR element is used for an MRAM, a thermal processat about 400° C. is performed in a semiconductor process of hydrogensintering or a passivation process.

[0007] However, when an element having an antiferromagnetic layer isheated to at least 300° C., the spin polarization of the magnetic layersdecreases due to diffusion of the Mn included in the antiferromagneticlayer, and the pinning magnetic field is decreased due to the change ofthe composition of the antiferromagnetic layer (see S. Cardoso et.al.,J.Appl.Phys. 87, 6058(2000)). Also, in elements in which a layeredferrimagnetic structure is layered on an antiferromagnetic material, thelayer structure of the layered ferrimagnetic structure is disturbedduring thermal processing, so that an increase of the pinning magneticfield cannot be expected. Thus, an improvement of the thermal resistanceis desired for magneto-resistive elements. An increase in the thermalresistance also is desired for CIP (current in plane)—GMR elements, inwhich the current flows in the film plane.

[0008] Furthermore, a high magnetic resistance change ratio still hasnot been attained at room temperatures with elements using half metals.In particular when forming an oxide half-metal material by sputteringwith an oxide target, the oxygen amount easily deviates from thestoichiometric ratio, and it is difficult to obtain high-qualitymagnetic thin films. But there is a possibility that higher magneticresistance change ratios can be obtained with magneto-resistive elementsusing half metals.

[0009] Furthermore, in particular in TMR elements, there is the problemthat there are sometimes large non-symmetries in the response toexternal magnetic fields.

SUMMARY OF THE INVENTION

[0010] According to a first aspect of the present invention, amagnetoresistive element includes an intermediate layer and a pair ofmagnetic layers sandwiching the intermediate layer, and one of themagnetic layers is a pinned magnetic layer in which magnetizationrotation with respect to an external magnetic field is harder than inthe other magnetic layer. The pinned magnetic layer includes at leastone non-magnetic film and magnetic films sandwiching the non-magneticfilms, and the magnetic films are magnetostatically coupled to oneanother via the non-magnetic film.

[0011] The magnetic films are magnetized antiparallel to one anotherwith the non-magnetic film arranged between them, and the magnetostaticenergy forms a closed magnetic circuit, that is, the magnetic films aremagnetostatically coupled, so that leaking magnetic fields causingmagnetic field shifts in the other magnetic layer (free magnetic layer)are suppressed. Also in layered ferrimagnetic structures utilizingantiferromagnetic coupling that have been used conventionally, themagnetization directions become antiparallel. However, layeredferrimagnetic structures utilize the RKKY effect(Ruderman-Kittel-Kasuya-Yoshida effect), so that they are very sensitiveto the thickness of the non-magnetic film. By contrast, when usingmagnetostatic coupling, the dependency on the thickness is relativelysmall. Furthermore, when magnetostatic coupling is used, thenon-magnetic film itself can be thick. Thus, the thermal stability ofthe element can be improved by using magnetostatic coupling.

[0012] According to a second aspect of the present invention, amagnetoresistive element includes an intermediate layer and a pair ofmagnetic layers sandwiching the intermediate layer, and one of themagnetic layers is a pinned magnetic layer in which magnetizationrotation with respect to an external magnetic field is harder than inthe other magnetic layer. The pinned magnetic layer includes at leastone non-magnetic film and magnetic films sandwiching the non-magneticfilm, and the magnetic films are coupled to one another by magnetostaticor antiferromagnetic coupling via the non-magnetic film, and when themagnetic films are magnetic films that are arranged at positions m (withm being an integer of 1 or greater) from the intermediate layer, Mm isan average saturation magnetization of the magnetic films m and dm istheir respective average film thickness, Mdo is the sum of the productsMm×dm of the magnetic films with odd m and Mde is the sum of theproducts Mm×dm of the magnetic films with even m, then

0.5<Mde/Mdo<1.

[0013] In this element, the magnetic films are magnetized antiparallelby antiferromagnetic or magnetostatic coupling, with non-magnetic filmsdisposed between them. To completely eradicate the magnetic fieldleaking from the pinned magnetic layer, Mde/Mdo should be set to 1.However, as the result of experiments explained below, it was found thatin particular in TMR elements, positive magnetic coupling occurs betweenthe pinned magnetic layer and the free magnetic layer. This couplingcauses non-symmetries in the response of the magnetic resistance toexternal magnetic fields. In these elements, it is more advantageous toset Mde/Mdo<1, so that a leaking magnetic field canceling the positivemagnetic coupling is generated (causing negative magnetic coupling),improving non-symmetries. When the leaking magnetic field is too large,non-symmetries occur on the negative coupling side, so that it ispreferable to set Mde/Mdo≧0.6.

[0014] According to a third aspect of the present invention, amagnetoresistive element includes an intermediate layer and a pair ofmagnetic layers sandwiching the intermediate layer. At least one of themagnetic layers includes an oxide ferrite having a plane orientationwith a (100), (110) or (111) plane, and a change in magnetic resistanceis detected by introducing an external magnetic field in the plane. Theexternal magnetic field is preferably introduced in a direction of theaxis of easy magnetization in the plane but the oxide ferrite can benon-orientated in the plane.

[0015] Examples of oxide ferrites include MnZn ferrite, NiZn ferrite andmagnetite (Fe₃O₄). When grown in an orientated state, oxide ferriteshave a relatively high magnetic resistance change ratio in teh (100),(110) or (111) plane. And when grown epitaxially, the magnetizationresponsiveness of the magnetic resistance changes with respect toexternal magnetic fields is increased by introducing an externalmagnetic field in the direction of the axis of easy magnetization.

[0016] Yet another aspect of the present invention provides a methodthat is suitable for manufacturing the elements as described above. Thismethod is suitable for manufacturing a magnetoresistive elementincluding an intermediate layer and a pair of magnetic layerssandwiching the intermediate layer, and at least one of the magneticlayers includes an oxide ferrite. The method includes forming the oxideferrite by sputtering with an oxide target while applying a bias voltageto a substrate including a plane on which the oxide ferrite is to beformed so as to adjust an amount of oxygen supplied to the oxide ferritefrom the oxide target.

[0017] When sputtering with oxide targets, tiny composition deviationseasily deteriorate the properties of the element. With theabove-described method, the composition control becomes easier, so thatthe reproducibility of the element increases. This method is alsosuitable for other compound magnetic thin films. That is to say,according to yet another aspect of the present invention, a method forforming a magnetic compound film is provided, which includes forming themagnetic compound film by sputtering with a compound target whileapplying a bias voltage to a substrate including a plane on which themagnetic compound film is to be formed so as to adjust the amount of atleast one selected from oxygen and nitrogen supplied to the magneticcompound film from the compound target. With this method, it is possibleto obtain compound magnetic thin films of the desired stoichoimetricratio with high reproducibility.

[0018] The present invention includes the element that can be describedfrom two or more of the aspects. The element of the present inventioncan include more layers, for example, two or more non-magnetic layersand magnetic layers sandwiching the non-magnetic layers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a diagram illustrating the magnetic field shift s.

[0020]FIG. 2 is a cross sectional view of a magneto-resistive elementaccording to one embodiment of the present invention.

[0021]FIG. 3 is a diagram illustrating a relationship between the Althickness for the intermediate film and the magnetic shift.

[0022] FIGS. 4A-C are X-ray analysis charts illustrating the differencesof the crystal structure of the Fe oxide obtained depending on the biason the substrate, namely at 0 W (FIG. 4A), at 5 W (FIG. 4B), and at 10 W(FIG. 4C).

[0023] FIGS. 5A-B show examples of an M-H curve and an MR curve whenapplying an external magnetic field in certain directions to Fe₃O₄formed on an MgO (100) plane. In FIG. 5A the external magnetic field isapplied from <100> axis direction, and in FIG. 5B from <110> axisdirection.

[0024] FIGS. 6A-B show examples of an M-H curve and an MR curve whenapplying an external magnetic field in certain directions to Fe₃O₄formed on an MgO (110) plane. In FIG. 6A the external magnetic field isapplied from <110> axis direction, and in FIG. 6B from <001> axisdirection.

[0025] FIGS. 7A-B show examples of an M-H curve and an MR curve whenapplying an external magnetic field in arbitrary direction to Fe₃O₄formed on an MgO (111) plane.

[0026] FIGS. 8A-C show anisotropic energy distributions in various Fe₃O₄planes. FIG. 8A shows the distribution in the (100) plane, FIG. 8B inthe (110) plane and FIG. 8C in the (111) plane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The following is a description of the preferred embodiments ofthe present invention.

[0028] In the magnetoresistive element with improved thermal resistanceprovided by the first aspect of the present invention, it is desiredthat the thickness of the non-magnetic film is adjusted appropriately toachieve static magnetic coupling between the magnetic films. Apreferable thickness of the non-magnetic film for making the staticmagnetic coupling significant to the antiferromagnetic coupling is atleast 1.5 nm and that for making the static magnetic coupling dominantis at least 2.6 nm, although these will depend on the non-magneticmaterial. When the thickness of the non-magnetic film exceeds 10 nm, themagnetic coupling accordingly becomes weaker.

[0029] In this element, it is preferable that that the element area isnot larger than 10 μm², more preferably not larger than 1 μm². Here,“element area” means the area in the intermediate layer that isperpendicular to the direction of the current flow, and in a verticalcurrent-type element, the area of the film surface of the intermediatelayer. If the element is made smaller to the point where this areabecomes 10 μm² or less, then the thickness of the magnetic films becomesrelatively large with respect to the area. Therefore, the demagnetizingfield per magnetic layer becomes large, and the magnetostatic energy perlayer becomes large. The magnetization directions of the magnetic layerstend to assume an antiparallel state in order to reduce themagnetostatic energy, so that an increase of the magnetostatic energystabilizes the antiparallel state of the magnetization. In order tostabilize the magnetization directions even further, it is also possibleto provide the contour shape of the pinned magnetic layer withanisotropy. As a favorable contour shape, it is preferable that theratio of long axis to short axis is 2 or greater. There is no particularlimitation to the contour shape, and it can be rectangular, rhombic orelliptical.

[0030] If a magneto-resistive element according to the second aspect ofthe present invention is used, then the magneto-resistive response canbe improved. More specifically, the absolute value of a magnetic fieldshift of the other magnetic layer (a free magnetic layer) can bedecreased to not more than 50% of a coercivity of the free magneticlayer, specifically to 20 Oe or less, further to 3 Oe or less, or evento substantially 0 Oe. Here, the magnetic field shift s is defined as

s=(H ₁ +H ₂)/2,

[0031] where H₁ and H₂ are the two magnetic fields at which themagnetization is zero (M=0) in the magnetization-magnetic field curve(M-H curve) showing the relationship between magnetic field (H) andmagnetization (M) (see FIG. 1).

[0032] In a magneto-resistive element, the two magnetic fields at whichthe resistance of the element is the average value of the maximum andthe minimum in the resistance-magnetic field curve substantiallycorrespond to H₁ and H₂, respectively.

[0033] In this element, the value of Mde/Mdo can be adjusted asappropriate to decrease the magnetic shift to the afore-mentionedlevels. Consequently, although the value of Mde/Mdo may differ dependingon the level of positive magnetic coupling in the element, it is usuallypreferable that the value of Mde/Mdo is about 0.5 to 0.9.

[0034] If the magnetic films are all soft magnetic layers, then themagnetization direction on the film tend to rotate easily in response toexternal magnetic fields. Therefore, it is preferable that at least oneof the magnetic films has a high coercive force, for example at least500 Oe (39.8 kA/m). Preferable high coercive materials are CoPt, FePt,CoCrPt, CoTaPt, FeTaPt and FeCrPt, for example.

[0035] It is also possible to stabilize the magnetizations coupled inantiparallel by using an antiferromagnetic layer. In this case, theelement of the present invention further includes an antiferromagneticlayer, leading to an element in which this antiferromagnetic layer ismagnetically coupled with the pinned magnetic layer. As theantiferromagnetic material, it is possible to use Cr-containingantiferromagnetic materials expressed by the composition formulaCr_(100−x)Me_(x) (wherein Me is at least one selected from Re, Ru andRh, and 0.1≦x≦20, atomic ratio) as well as Mn-containingantiferromagnetic materials such as FeMn and IrMn. PreferableMn-containing antiferromagnetic materials include noble metal-basedantiferromagnetic materials expressed by the composition formulaMn_(100−x)Me_(x) (wherein Me is at least one selected from Pd and Pt,and 40≦x≦55).

[0036] In order to increase the crystallinity of the antiferromagneticmaterial, it is possible to form the antiferromagnetic layer on a primerlayer having a crystal structure and/or a lattice constant similar tothose of the antiferromagnetic material. For example, if theantiferromagnetic material is PtMn or PtPdMn, then NiFe or NiFeCr can beused for the primer layer.

[0037] The following is an example of a preferable embodiment of amagneto-resistive element in accordance with a third aspect of thepresent invention. In the case of magnetite grown epitaxially in the(110) plane on a substrate or a primer layer, changes in magneticresistance can be detected by introducing an external magnetic field inthe range of 30° to 150° in the (110) plane, taking the <100> axisdirection in that plane as 0 degrees. When an external magnetic field isintroduced in this manner, the magnetization responsiveness of themagnetic resistance changes increases. An oxide ferrite in which, of thecrystal magnetic anisotropic energies, at least K₁ is negative andpreferably also K₂ is negative is suitable for this embodiment. In thecase of an oxide ferrite in which K₁ is positive and preferably also K₂is positive, an external magnetic field in the range of 170° to 190° inthe (110) plane should be introduced, taking the <100> axis direction as0 degrees.

[0038] In the case of magnetite that has been grown epitaxially in the(100) plane, an external magnetic field in the range of 40° to 50° or130° to 140° in the (100) plane should be introduced, taking the <100>axis direction in the (110) plane as 0 degrees. An oxide ferrite inwhich, of the crystal magnetic anisotropic energies, K₁ is negative andpreferably also K₂ is negative is suitable for this embodiment. In thecase of an oxide ferrite in which K₁ is positive and preferably also K₂is positive, an external magnetic field in the range of 85° to 95° or175° to 185° in the (110) plane should be introduced, taking the <100>axis direction as 0 degrees.

[0039] In the case of magnetite that has been grown epitaxially in the(111) plane, a high magnetization responsiveness is attained bydetecting changes in the magnetic resistance by introducing an externalmagnetic field within an arbitrary angular range within that plane.Taking the <100> axis direction as 0 degrees, it is even better tointroduce an external magnetic field from an angle of a range including30°, 90° or 150° in the (111) range, preferably substantially one of theangular ranges mentioned above. These preferable embodiments share anoxide ferrite in which at least K₂ is negative. If at least K₂ isnegative, then, taking the <110> axis direction as 0 degrees, it is evenbetter to introduce an external magnetic field from an angle of a rangeincluding 0°, 60° or 120°, preferably substantially one of the angularranges mentioned above.

[0040] For magnetite grown aligned in the (100) plane, the (110) planeor the (111) plane, and for magnetite grown without orientation, theexternal magnetic field should be introduced at an angle within anarbitrary range in these planes. In particular, if the average crystalwidth of the in-plane direction of Fe₃O₄ is not more than 10 nm, thenthe apparent crystal magnetic anisotropic energy becomes small, so thatit is possible to take magnetically soft Fe₃O₄ or a ferromagneticmaterial having Fe₃O₄ as its principal component. This is not limited toFe₃O₄, but is true for all oxide ferrites.

[0041] It has been found that the change rate of the magnetic resistancetends to grow when an element including a d-electron is included in theintermediate layer adjacent to the oxide ferrite. Elements includingd-electrons are the elements with an atom number of 21 or greater in theperiodic table of elements.

[0042] If a compound magnetic thin film including oxygen and/or nitrogenand a transition metal, such as oxide ferrite, is formed by sputteringwith a compound magnetic target, then the oxygen or nitrogen content inthe composition tends to deviate. However, if the compound magnetic thinfilm is formed while applying a bias voltage to the substrate or primeronto which it is formed to control the oxygen and/or nitrogen includedin the thin film, then it is possible to form the compound magnetic thinfilm with high reproducibility. This method also can be combined withreactive sputtering using a sputtering gas including an inert gas andoxygen and/or nitrogen.

[0043] The application of a bias voltage can be accomplished by:

[0044] 1. Electrically insulating (floating) the substrate from ground,and controlling the applied bias voltage with the plasma density, whichis determined by the discharge power, gas pressure, etc.;

[0045] 2. Electrically insulating (floating) the substrate from ground,and applying a dc or a high-frequency (RF) bias voltage with an externalpower source. The RF bias frequency should be in the ordinarily usedrange, for example 10 MHz or higher.

[0046] This method is suitable for RF sputtering, such as RF magnetronsputtering. When applied to these sputtering methods, the film formationcan be performed by applying a dc or RF bias voltage to the substratewhile applying an RF voltage to the compound magnetic material taken asthe target. It is preferable that the supply of RF voltage to the targetand the substrate is synchronized, so as to control the formation of amagnetic deterioration layer on the uppermost film surface.

[0047] This film formation method is particularly suitable for theformation of oxide magnetic thin films of oxide ferrite or the like.Generally, the oxide magnetic target has a relatively high electricalresistance, and when it is used for film formation without bias, toomuch oxygen tends to be supplied. In order to reduce the oxygen in thefilm, it is advantageous to apply a negative bias voltage, and since theelectrical resistance of the film is high, the application of an RF biasvoltage is preferable. It should be noted, however, that the method formanufacturing the above-described element including oxide ferrite is notlimited to the above-described film forming method. For example, it isalso possible to use a compound magnetic target in which the oxygenamount has been set below the stoichiometric ratio utilizing thecomposition deviation in ordinary sputtering. It is further possible touse the above-described target and supplement the lacking oxygen byreactive sputtering.

[0048] To increase the crystallinity of the compound magnetic thin film,the substrate temperature should be 250° C. to 700° C. Since a biasvoltage is applied, radiative heating is suitable to heat the substrate.

[0049] The magneto-resistive element explained above is particularlyuseful for perpendicular current-type elements (CPP-GMR elements, TMRelements), in which the current flows perpendicular to the films in amultilayered film, but it is also effective for elements in which thecurrent flows in the film plane (CIP-GMR elements).

[0050] For the intermediate layer of a TMR element, it is possible touse a semiconductor or an insulating material—including at least oneelement selected from oxygen, nitrogen, carbon and boron. Examples ofpreferable materials include SiO₂, SiC, Si₃N₄, Al₂O₃, AlN, Cr₂O₃, TiC,HfO₂, HfN, HfC, Ta₂O₅, TaN, TaC, BN, B₄C, DLC (diamond-like carbon), C₆₀and mixtures thereof.

[0051] For the intermediate layer of a GMR element, it is possible touse a semiconductor including a transition metal. It is also possible touse a conductive compound including a transition metal and at least oneselected from oxygen, nitrogen and carbon. In the case of a CPP-GMRelement, the element area (that is, the area of the intermediate layerthrough which current flows) should be not more 0.1 μm. This is becauserestricting the element area increases the electrical resistance of theelement at the same time as its thermal resistance. It is particularlypreferable to use at least one selected from V, Nb, Ta, Cr, Mo, W, Cu,Ag, Au, Ru, Rh, Ir, Re and Os for the intermediate layer. As long as theconductivity of these metals is not lost, it is also possible to usethem in the form of oxides, nitrides or carbides. It is further possibleto use a mixture of a transition metal X and a compound R (at least oneselected from SiO₂, SiC, Si₃N₄, Al₂O₃, AlN, Cr₂O₃, Cr₂N, TiO, TiN, TiC,HfO₂, HfN, HfC, Ta₂O₅, TaN, TaC, BN and B₄C). Furthermore, sometimes itis also possible to raise the electrical resistance and the thermalresistance of the element by devising a multilayer film with at leasttwo layers, such as X/R.

[0052] For the non-magnetic film, a non-magnetic conductive materialshould be used. Examples of preferable materials for a non-magnetic filmthrough which the magnetic films are magnetostatically coupled includeTi, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Al, Rh, Pt and Pd. Examples ofpreferable materials for a non-magnetic film through which the magneticfilms are coupled by antiferromagnetic coupling include Cr, Cu, Ag, Au,Ru, Rh, Ir, Re and Os.

[0053] While the thickness of the non-magnetic film that is preferablefor antiferromagnetic coupling also depends on the material, it isroughly 0.2 to 1.3 nm. If the non-magnetic material is for example Cr,then this thickness is preferably 0.8 to 1.3 nm, whereas for Ru it ispreferably 0.2 to 0.5 nm or 0.6 to 1.0 nm, for Ir it is preferably 0.3to 0.5 nm, and for Rh it is preferably 0.4 to 0.8 nm.

[0054] There is no particular limitation to the material and thethickness of the magnetic films, and it is appropriate to apply thematerials and thicknesses that are used conventionally. The thickness ofthe magnetic film suitable for the magnetostatic coupling is 1.5 to 20nm. The thickness less than 1.5 nm reduces lowering of magnetostaticenergy while the thickness more than 20 nm may prevent leaking magneticfields from coupling.

[0055] It is preferable that the magnetic layers are made of thefollowing materials at least in the region within 0.5 nm from theinterface with the intermediate layer:

[0056] 1. Co-based amorphous materials such as CoNbZr, CoTaZr, CoFeB,CoTi, CoZr, CoNb, CoMoBZr, CoVZr, CoMoSiZr, CoMoZr, CoMoVZr and CoMnB;

[0057] 2. Fe-based microcrystal materials, such as FeSiNb or Fe(Si, Al,Ta, Nb, Ti)N;

[0058] 3. Materials including at least 50 wt % of a ferromagneticelement selected from Fe, Co and Ni, for example FeCo alloys, NiFealloys, NiFeCo alloys, or ferromagnetic materials such as FeCr, FeSiAl,FeSi, FeAl, FeCoSi, FeCoAl, FeCoSiAl, FeCoTi, Fe(Ni)(Co)Pt,Fe(Ni)(Co)Pd, Fe(Ni)(Co)Rh, Fe(Ni)(Co)Ir, or Fe(Ni)(Co)Ru, or dilutemagnetic alloys;

[0059] 4. Nitrides such as FeN, FeTiN, FeAlN, FeSiN, FeTaN, FeCoN,FeCoTiN, FeCoAlN, FeCoSiN, and FeCoTaN;

[0060] 5. Fe₃O₄;

[0061] 6. Half-metallic materials such as XMnSb (wherein X is at leastone selected from Ni, Cu and Pt), LaSrMnO, LaCaSrMnO, CrO₂;

[0062] 7. Spinel oxides such as perovskite oxides, MnZn ferrite and NiZnferrite; or

[0063] 8. Garnet oxides.

[0064] Also possible is a ferromagnetic or ferrimagnetic materialincluding at least 50 wt % of these materials. It should be noted thatthroughout this specification, the elements in parentheses are optionalelements.

[0065]FIG. 2 shows an example of the configuration of the elementsprovided by the present invention. In this element, a lower electrodealso serving as a primer layer 13, a first magnetic layer 17, anintermediate layer 16, a second magnetic layer 15 and an upper electrode11 are layered in that order on a substrate 14. A mesa-shaped elementportion made of the magnetic layers and the intermediate layer isenclosed by an inter-layer insulating film 12. Either one of the firstand second magnetic layers can serve as the free magnetic layer (or thepinned magnetic layer), and either one of the magnetic layers caninclude an oxide ferrite. The magnetic layers and the intermediate layeralso can be multilayer films, and it is further possible to add otherlayers, such as an antiferromagnetic layer. The configuration of theelement is not restricted to the example shown in FIG. 2.

[0066] The magnetic and other layers or films can be formed by anysuitable gas-phase film forming method known in the art. Examples ofsuitable methods include ion beam deposition (IBD), cluster ion beamdeposition, or sputtering methods, such as RF, DC, ECR (electroncyclotron resonance), helicon, ICP (inductively coupled plasma)sputtering or sputtering with opposing targets, MBE (molecular beamepitaxy), or ion plating. In addition to these PVD methods, it is alsopossible to use CVD (chemical vapor deposition) methods, in particularfor making the inter-layer insulating film.

[0067] The intermediate layer, which is a compound such as an oxide, canbe formed directly, if using chemical beam epitaxy, gas source MBE,reactive vapor deposition, reactive sputtering or the like. If theintermediate layer is formed by a method in which a plasma is generated(for example reactive sputtering), then a barrier layer should be formedbeforehand on the magnetic layer, to suppress the oxidation of theexposed magnetic layer. As a barrier layer, an extremely thin layer of,for example, one to several atoms of Al, Si, Ta, Hf, Nb, V or Cr issuitable. In reactive vapor deposition, in which no plasma is generated,it is also possible to protect the magnetic layer by forming, forexample, an oxide, nitride, carbide or boride layer of about one atomthickness. Instead of directly forming the intermediate layer, which isa compound, it is also possible to form an element contained in theintermediate layer (for example Al) on the magnetic layer, and form thecompound (for example Al₂O₃) by exposing this element to an atmospherewith atoms, molecules, ions (plasma), or radicals of a reactive gasincluding oxygen for example, at a suitable pressure and reactiontemperature, and for a suitable time. It is also possible to form anintermediate layer of the desired thickness by repeating the process offilm formation and oxidation or the like.

[0068] There is no particular limitation to the method for processingthe element portion into a mesa shape, and it can be performed by anyprocess that is ordinarily used for microprocessing, for examplephysical or chemical etching, such as ion milling, RIE, EB, or FIBetching, or photolithography techniques. In order to make the lowerelectrode flat, it is also effective to process the surface by CMP orcluster ion beam etching to increase the magnetic resistance changeratio.

WORKING EXAMPLES Working Example 1

[0069] The following samples were produced by magnetron sputtering on athermally oxidized Si substrate.

[0070] Sample 1:Ta(3)/Cu(50)/Ta(3)/CoFe(3)/Al₂O₃(1)/CoFe(3)/Ru(0.8)/CoFe(3)/PtMn(20)/Ta(3)

[0071] Sample 2: Ta(3)/Cu(50)/Ta(3)/CoFe(3)/Al₂O₃(1)/CoFe(7)/Ta(3)

[0072] Sample 3.Ta(3)/Cu(50)/Ta(3)/CoFe(3)/Al₂O₃(1)/CoFe(3)/Ta(3)/CoFe(10)/Ta(3)

[0073] Sample 4.Ta(3)/Cu(50)/Ta(3)/CoFe(3)/Al₂O₃(1)/CoFe(3)/Ta(3)/CoPt(4.4)/Ta(3)

[0074] Sample 5.Ta(3)/Cu(50)/Ta(3)/CoFe(3)/Al₂O₃(1)/CoFe(3)/Ta(3)/CoFe(3)/PtMn(20)/Ta(3)

[0075] Sample 6:Ta(3)/Cu(50)/Ta(3)/CoFe(3)/Cu(2.2)/CoFe(3)/Ta(3)/CoFe(3)/PtMn(20)/Ta(3)

[0076] The numbers in parentheses denote the film thicknesses (in nm;this is also true in the following). Here, Ta(3)/Cu(50)/Ta(3) serves asa lower electrode and primer layer,CoFe(3)/Ru(0.8)/CoFe(3)/PtMn(20)/Ta(3) is a pinned layer with layeredferrimagnetic structure, CoFe(3)/Ta(3)/CoPt(4.4), CoFe(3)/Ta(3)/CoFe(3)and CoFe(3)/Ta(3)/CoFe(10) are pinned magnetic layers usingmagnetostatic coupling, Al₂O₃ and Cu are intermediate layers, and therest are free magnetic layers (except the outermost layer of Ta(3),which is a protective film). The coercive force of the CoPt(4.4) is 500Oe.

[0077] After forming these films, a thermal process is performed for 1.5hours at 400° C. in a magnetic field of 5 kOe (398 kA/m). Next, usingsteppers, the element area, which is the area through which the currentflows in the intermediate layer, is micro-processed to a mesa shape of0.1 to 20 μm² with an aspect ratio of 4:1. Subsequently, the inter-layerinsulating film and the upper electrode were formed, yielding a verticalcurrent-type magneto-resistive element. It should be noted that thelongitudinal direction of the element was set to the direction in whichthe magnetic field was applied during thermal processing.

[0078] Table 1 shows the magnetic resistance change ratio (MR value)measured by applying an external magnetic field of ±1000 Oe (79.6 kA/m)in the longitudinal direction of the resulting elements. TABLE 1 elementarea (μm²) Sample No. 0.1 0.5 2 10 20 1 16 17 18 19 20 2 25 21 18 15 123 35 31 23 20 17 4 36 35 29 27 26 5 40 39 34 32 29 6 27 25 22 20 13

[0079] Compared to Sample 1, which includes a layered ferrimagneticstructure, and Sample 2, which uses a coercive force difference due toshape anisotropies, it can be seen that at element areas of 10 μm² orless, Samples 3 to 6 had higher MR values. It seems that the MR valuesof Samples 3 to 6 are higher than that of Sample 2, because theinfluence of the magnetic field leaking into the free magnetic layer isdecreased by the magnetostatic coupling. In Samples 4 and 5, anantiferromagnetic material or a high coercivity material is used, sothat there is only a small dependency of the MR value on the elementarea. Sample 6, which uses Cu for the intermediate layer, includes anantiferromagnetic material, but as the element area becomes smaller, thecurrent per area effectively increases, so that the dependency on theelement area is large.

[0080] Then, the MR value when changing the film thickness X of the Taof the non-magnetic film was examined for with the following filmconfiguration. Table 2 shows the results. The conditions under whichthese elements were prepared were similar to the above, including theparameters for the thermal processing, and the element aspect ratio wasalso 4:1. The element area was set to 0.1 μm².

[0081] Sample 7:Ta(3)/Cu(50)/Ta(3)/CoFe(3)/Al₂O₃(1)/CoFe(3)/Ta(X)/CoFe(3)/PtMn(20)/Ta(3)TABLE 2 Ta film thickness (nm) MR (%) 1 20 1.5 35 3 40 5 41 10 35 20 25

[0082] High MR values are attained when the film thickness of thenon-magnetic film is in a range in which the magnetostatic coupling isdominant (preferably about 2.6 to 10 nm). As a result of similarexperiments, it was found that a preferable thickness for the magneticlayers is in the range of 1.5 to 20 nm. Furthermore, a similar tendencyis also observed in similar experiments with the ferromagneticmaterials, non-magnetic materials and high coercivity materials(antiferromagnetic materials) mentioned above.

Working Example 2

[0083] An element having the following film configuration was producedon a thermally oxidized Si substrate.

[0084]Ta(3)/Cu(500)/Ta(3)/CoFe(3)/Ru(0.7)/CoFe(3)/Al₂O₃/CoFe(3)/NiFe(4)/Ta(3)

[0085] Changing the film thickness of the Al that is applied for formingthe Al₂O₃ layer, the respective M-H curves were measured to determinethe magnetic field shifts. The results are shown in FIG. 3. As the filmthickness of the Al becomes thinner, the magnetic field shifts increase.The reason for this is not entirely clear, but it seems that as theAl₂O₃ layer becomes thinner, orange peel coupling between the freemagnetic layer and the pinned magnetic layer causes positive magneticcoupling between the two magnetic layers.

[0086] Next, setting the Al film thickness to 0.7 nm, elements havingthe following film configuration were produced.

[0087] Sample 11:Ta(3)/Cu(500)/Ta(3)/CoFe(3)/Ru(0.7)/CoFe(5)/Al₂O₃/CoFe(3)/NiFe(4)/Ta(3)

[0088] Sample 12:Ta(3)/Cu(500)/Ta(3)/CoFe(3)/Ru(0.7)/CoFe(3)/Al₂O₃/CoFe(3)/NiFe(4)/Ta(3)

[0089] When the M-H curves of these two elements were measured, it wasfound that the magnetic field shift was suppressed in Sample 11, buttended to increase in Sample 12. When the multilayer film in Sample 11was processed into a mesa shape and the MR value was measured, an MRvalue of 30% was obtained at an RA (normalized bonding resistance) of 15Ωμm². The magnetic field shift was suppressed to substantially 0 Oe.

[0090] Thus, especially when the film thickness of the intermediatelayer is thin, magnetic field shifts are suppressed when the product offilm thickness and saturation magnetization of the magnetic films on theside of the intermediate layer is large. As a result of even moredetailed experimentation, it was found that when Mde/Mdo is more than0.5 and less than 1, a magneto-resistive element with little magneticshift can be obtained.

Working Example 3

[0091] Using an Fe₃O₄ target, an Fe oxide film was produced by RFmagnetron sputtering at room temperature on a thermally oxidized Sisubstrate. During the film formation, an RF bias of 0, 5 or 10 W wasapplied. FIGS. 4A to 4C show the results of the X-ray analysis of theresulting Fe oxide films. Fe₂O₃ was formed at an RF bias of 0 W, Fe₃O₄was formed at 5 W, and FeO was formed at 10 W. Thus, as the biasincreased, the amount of oxygen decreased. Observing the (111) planeparallel to the substrate surface, it could be confirmed that the Fe₃O₄oriented itself in the (111) plane. Moreover, from measuring the M-Hcurve, it was found that the Fe₃O₄ was not oriented inside the filmsurface. Forming Fe₃O₄ at different substrate temperatures, it was foundthat it is easy to form Fe₃O₄ with high crystallinity in a substratetemperature range of 250° C. to 700° C.

[0092] Setting the substrate temperature to 300° C. and forming a Ptfilm of 300 nm film thickness on the thermally oxidized Si substrate, anFe₃O₄ film of 50 nm film thickness was formed at a bias of 5 W. Then,after returning the substrate temperature to room temperature, an Al₂O₃film was formed, and a CoFe film of 20 nm film thickness was layered ontop of that. The MR value of this multilayer film was measured to beabout 3%. This value was constant, regardless of the direction of theapplied magnetic field.

[0093] Multilayer films were formed, as described above, on the (100),(110) and (111) planes of an MgO substrate. The M-H curves of themultilayer films and the MR curves after microprocessing them are shownin FIGS. 5 to 7. FIGS. 5A to 5B show the results when applying anexternal magnetic field in <100> or <010> axis direction in the (100)plane, FIGS. 6A to 6B show the results when applying an externalmagnetic field in <110> or <001> axis direction in the (110) plane, andFIGS. 7A to 7B show the results when applying an external magnetic fieldof arbitrary orientation in the (111) plane.

[0094] The highest MR was obtained when applying an external magneticfield in <110> axis direction in the (110) plane. This result suggeststhat the highest MR is attained when applying an external magnetic fieldin the direction regarded as the axis of easy magnetization, as can beseen from the anisotropic energy distribution charts (FIGS. 8A to 8C) ofthe films. When using for a device a material that has a relativelylarge crystal magnetic anisotropy while having a high spin polarization,such as Fe₃O₄, then the material does not undergo magnetic saturation inpractical magnetic field ranges when using the axis of hardmagnetization. Therefore, it becomes difficult to attain a high MR.

[0095] Moreover, as shown by experimentation as described above, the MRis further improved when using an intermediate layer containing elementswith d electrons (such as Ta) in the portion adjacent to the oxideferrite.

[0096] With the present invention, thermal resistance is improved byintroducing magnetostatic coupling into the pinned magnetic layer of themagnetoresistive element. Moreover, with the present invention, magneticfield shifts are reduced by generating negative coupling in the magneticlayers of the pinned magnetic layer of the magnetoresistive element.Furthermore, with the present invention, a magnetoresistive element withhigh MR can be provided by specifying the direction in which an externalmagnetic field is applied to the oxide ferrite. And, with the presentinvention, compound magnetic thin films with excellent properties, suchas oxide ferrite films, can be formed with high reproducibility byadjusting the amount of oxygen or the like by applying a bias voltage tothe substrate.

[0097] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof Theembodiments disclosed in this application are to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than by the foregoingdescription, all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

What is claimed is:
 1. A magnetoresistive element, comprising: an intermediate layer; and a pair of magnetic layers sandwiching the intermediate layer; wherein one of the magnetic layers is a pinned magnetic layer in which magnetization rotation with respect to an external magnetic field is harder than in the other magnetic layer; wherein the pinned magnetic layer includes at least one non-magnetic film and magnetic films sandwiching the non-magnetic film; and wherein the magnetic films are magnetostatically coupled to one another via the non-magnetic film.
 2. The magneto-resistive element according to claim 1, wherein an element area, which is the area of the intermediate layer in a plane perpendicular to the direction in which current flows, is not more than 10 μm².
 3. The magneto-resistive element according to claim 1, wherein at least one of the magnetic films has a coercivity of at least 500 Oe.
 4. The magneto-resistive element according to claim 1, further comprising an antiferromagnetic layer, which is magnetically coupled with the pinned magnetic layer.
 5. The magneto-resistive element according to claim 1, wherein the intermediate layer is made of a semiconductor or an insulator and includes at least one element selected from oxygen, nitrogen, carbon and boron.
 6. The magneto-resistive element according to claim 1, wherein the intermediate layer is made of a conductive material including a transition metal.
 7. The magneto-resistive element according to claim 6, wherein the element area, which is the area of the intermediate layer in a plane perpendicular to the direction in which current flows, is not larger than 0.1 μm².
 8. A magnetoresistive element, comprising: an intermediate layer; and a pair of magnetic layers sandwiching the intermediate layer; wherein one of the magnetic layers is a pinned magnetic layer in which magnetization rotation with respect to an external magnetic field is harder than in the other magnetic layer; wherein the pinned magnetic layer includes at least one non-magnetic film and magnetic films sandwiching the non-magnetic film; wherein the magnetic films are coupled to one another by magnetostatic or antiferromagnetic coupling via the non-magnetic film; and when the magnetic films are magnetic films that are arranged at positions m (with m being an integer of 1 or greater) from the intermediate layer, Mm is an average saturation magnetization of the magnetic films m and dm is their respective average film thickness, Mdo is the sum of the products Mm×dm of the magnetic films with odd m and Mde is the sum of the products Mm×dm of the magnetic films with even m, then 0.5<Mde/Mdo<1.
 9. The magneto-resistive element according to claim 8, wherein an absolute value of a magnetic field shift of said other magnetic layer that is a free magnetic layer is not more than 50% of a coercivity of the free magnetic layer, where the magnetic field shift is given by the equation s=(H ₁ +H ₂)/2,  wherein H₁ and H₂ (with H₁>H₂) are two magnetic fields at which magnetization becomes zero (M=0) in a magnetization-magnetic field curve (M-H curve) showing the relationship between magnetic field (H) and magnetization (M).
 10. The magneto-resistive element according to claim 8, wherein at least one of the magnetic films has a coercivity of at least 500 Oe.
 11. The magneto-resistive element according to claim 8, further comprising an antiferromagnetic layer, which is magnetically coupled with the pinned magnetic layer.
 12. The magneto-resistive element according to claim 8, wherein the intermediate layer is made of a semiconductor or an insulator and includes at least one element selected from oxygen, nitrogen, carbon and boron.
 13. The magneto-resistive element according to claim 8, wherein the intermediate layer is made of a conductive material including a transition metal.
 14. The magneto-resistive element according to claim 13, wherein the element area, which is the area of the intermediate layer in a plane perpendicular to the direction in which current flows, is not larger than 0.1 μm².
 15. A magnetoresistive element, comprising: an intermediate layer; and a pair of magnetic layers sandwiching the intermediate layer; wherein at least one of the magnetic layers includes an oxide ferrite having a plane orientation with a (100), (110) or (111) plane; and wherein a change in electric resistance is detected by introducing an external magnetic field in said plane.
 16. The magneto-resistive element according to claim 15, wherein the external magnetic field is introduced in a direction of the axis of easy magnetization in said plane.
 17. The magneto-resistive element according to claim 16, wherein the oxide ferrite is oriented in the (110) plane, and, taking the direction of the <100> axis in that plane as 0°, the external magnetic field is introduced at an angle in a range of at least 30° and at most 150° in that (110) plane.
 18. The magneto-resistive element according to claim 16, wherein the oxide ferrite is oriented in the (100) plane, and, taking the direction of the <100> axis in that plane as 0°, the external magnetic field is introduced at an angle in a range of at least 40° and at most 50° or at least 130° and at most 140° in that (100) plane.
 19. The magneto-resistive element according to claim 16, wherein the oxide ferrite is aligned in the (111) plane and the external magnetic field is introduced in that (111) plane.
 20. The magneto-resistive element according to claim 15, wherein the oxide ferrite is non-orientated in said plane.
 21. The magneto-resistive element according to claim 15, wherein the oxide ferrite is magnetite.
 22. The magneto-resistive element according to claim 15, wherein the intermediate layer is made of a semiconductor or an insulator and includes at least one element selected from oxygen, nitrogen, carbon and boron.
 23. The magneto-resistive element according to claim 15, wherein the intermediate layer is made of a conductive material including a transition metal.
 24. The magneto-resistive element according to claim 23, wherein the element area, which is the area of the intermediate layer in a plane perpendicular to the direction in which current flows, is not larger than 0.1 μm².
 25. A method for manufacturing a magnetoresistive element comprising an intermediate layer and a pair of magnetic layers sandwiching the intermediate layer, wherein at least one of the magnetic layers includes an oxide ferrite; the method comprising: forming the oxide ferrite by sputtering with an oxide target while applying a bias voltage to a substrate including a plane on which the oxide ferrite is to be formed so as to adjust an amount of oxygen supplied to the oxide ferrite from the oxide target.
 26. The method for manufacturing a magneto-resistive element according to claim 25, wherein the applied bias voltage is a high-frequency bias voltage.
 27. The method for manufacturing a magneto-resistive element according to claim 25, wherein the substrate temperature is at least 250° C. and at most 700° C.
 28. A method for forming a magnetic compound film, the method comprising: forming the magnetic compound film by sputtering with a compound target while applying a bias voltage to a substrate including a plane on which the magnetic compound film is to be formed so as to adjust the amount of at least one selected from oxygen and nitrogen supplied to the magnetic compound film from the compound target.
 29. The method for forming a magnetic compound film according to claim 28, wherein the applied bias voltage is a high-frequency bias voltage.
 30. The method for forming a magnetic compound film according to claim 28, wherein the substrate temperature is at least 250° C. and at most 700° C. 